Why does it take 21 days to break a habit?

The 21-day habit-formation myth dates back to a misinterpretation of a 1960's book 'Psycho-Cybernetics' by Maxwell Maltz. In his book, Maltz discussed how it takes the same amount of time for the body to regenerate. However, there is no scientific evidence to back Maltz's claim for habit formation. Breaking a habit requires more persistence and consistency